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BIOACTIVITY OF EXTRACTS OF SOME NIGERIAN PLANTS ON INSECT PESTS OF STORED GRAINS

BY

ABIODUN AKINPELU DENLOYE

ABSTRACT

Five of the plant species used by farmers in southwestern Nigeria, Chenopodium ambrosioides (L), Tephrosia vogelii (Hook), Allium sativum (Linn), Allium fistulosum (Linn) and Zanthoxylum zanthoxyloides (Linn) were tested for their bioactivity against Sitophilus zeamais Motsch, Callosobruchus maculatus F., and Tribolium casteneum Herbst in laboratory and field bioassays. The selected plants were tested as powders, aqueous extract, ethanol extracts and volatile essential oils for toxicity, ovicidal, antiovipositional, and larvicidal effects as admixtures, dipped grain treatments or fumigants. On the basis of 48 hr LC₅₀ values Z. zanthoxyloides powder was the most toxic to S. zeamais (0.012 g/kg), T. casteneum (0.041 g/kg) and C. maculatus (0.05 g/kg) followed by C. ambrosioides with 0.49 g/kg (S. zeamais), 1.634 g/kg (C. maculatus) and 2.57 g/kg (T. casteneum) 48 h LC₅₀ values.  Ethanol extract of all test plant species were more toxic than their equivalent aqueous extracts. Values of the 48 h LC₅₀ show that Z. zanthoxyloides ethanol extract was more toxic to C. maculatus (0.021 g/L) than S. zeamais (0.035 g/L) or T. casteneum (0.085 g/L). Insecticidal activity of Z. zanthoxyloides ethanol extract persisted for 14 days post-treatment whereas there was measurable loss of activity for C. ambrosioides and T. vogelii respectively at 24 h post-treatment while A. sativum and A. fistullosum activity was lost as from 12 h post treatment. The essential oils of A. sativum, A. fistullosum and C. ambrosioides demonstrated fumigant toxicity against C. maculatus with a 24 hr LC₅₀ value of 1.33 μl/L) and S. zeamais (1.90 μl/L). Grain treatment with ethanol extracts resulted in significant reduction (p > 0.05) in the number of eggs laid by C. maculatus; fewer eggs were laid in Z. zanthoxylum-treated than Chenopodium extract treated grains. Only C. ambrosioides volatile oil gave appreciable ovicidal action against eggs of C. maculatus on cowpea (LC₅₀ = 2.066 μl/L).   Zanthoxylum zanthoxyloides gave protection from insect infestation in field tests for up to 180 days post-treatment just as pirimiphos methyl (Actellic) while protection with C. ambrosioides failed 90 days post treatment in treated cowpea and maize grains. Grain treatment did not result in loss of viability in Z. zanthoxyloides – treated grains but treatments in C. ambrosioides – treated grains did. The results obtained from this study show that the test plant species especially Z. zanthoxyloides, C. ambrosioides and T. vogelii can be exploited as sources of locally available alternative insecticides for the protection of grains against attack by C. maculatus and S. zeamais. The study further shows that the volatile oil of C. ambrosioides can be used as fumigant to reduce the number of eggs laid by C. maculatus or reduce their hatchability thus reducing the number of offspring in the succeeding generation.

 

ABSTRACT OF THESIS SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, UNIVERSITY OF LAGOS IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY (Ph. D) IN ZOOLOGY (2008)